ivi-shell: fix free in get_layers_under_surface

If a controller requests the layers under a surface that has no views attached,
Weston crashes since it tries to free the array that would be used to return the
found layers, but has not been allocated before.

Free the ppArray only if it was allocated in ivi_layout_get_layers_under_surface
before and no layers were found.

While at it, make it obvious that checking the length is an integer comparison
by comparing it to 0.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>